In recent years, the CMIs have been widely used in flexible AC transmission systems (FACTS) such as static synchronous series compensators (SSSC), dynamic voltage restorers (DVR), static synchronous compensators (STATCOM), and transformer-less unified power flow controllers (UPFC).
The CMIs are controlled by modulation. Modulation for the CMIs can be accomplished by either (1) fundamental frequency modulation (FFM) or (2) high-frequency pulse width modulation (PWM). Fundamental frequency modulation has much lower switching loss compared to the high-frequency PWM. As such, FFM is attractive for transmission-level UPFCs and other high-voltage and high-power applications.
However, one main drawback of traditional FFM is the unequal active/reactive power distribution among the H-bridge modules due to unequal pulse-width of output voltages. Such unequal distribution causes each H-bridge module to exhibit different DC capacitor currents and DC-link ripples. Hence, utilization of DC-link capacitors with traditional FFM is ineffective. This leads to either excessive voltage stress across insulated-gate bipolar transistors (IGBTs) or over-rated capacitor design in the H-bridge modules.
With traditional FFM, some H-bridge modules have greater duty cycles than others. The H-bridge modules with greater duty cycles have higher DC capacitor current and voltage ripples. One way to withstand higher capacitor currents and reduce the voltage ripples is to increase the DC-link capacitance for these H-bridge modules. However, increasing the DC-link capacitance is not cost-effective and is against modular design principles, in which all the H-bridge modules are expected to have the same type of capacitors having the same capacitance.